Semiconductor device and method of manufacturing such a device

ABSTRACT

The invention relates to a semiconductor device with a substrate ( 11 ) and a semiconductor body ( 12 ) with a heterojunction bipolar, in particular npn, transistor with an emitter region ( 1 ), a base region ( 2 ) and a collector region ( 3 ), which are provided with, respectively, a first, a second and a third connection conductor ( 4, 5, 6 ), and wherein the bandgap of the base region ( 2 ) is smaller than that of the collector region ( 3 ) or of the emitter region ( 1 ), for example by the use of a silicon-germanium mixed crystal instead of pure silicon in the base region ( 2 ). Such a device is characterized by a very high speed, but its transistor shows a relatively low BVeeo. In a device ( 10 ) according to the invention the doping flux of the emitter region ( 1 ) is locally reduced by a further semiconductor region ( 20 ) of the second conductivity type which is embedded in the emitter region ( 1 ). In this way, on the one hand, a low-impedance emitter contact is ensured, while locally the Gummel number is increased without the drawbacks normally associated with such an increase. In this way, the hole current in the, npn, transistor is increased and thus the gain is decreased. The relatively high gain of a Si—Ge transistor is responsible for the low BVCeOf which is consequently avoided in a device ( 10 ) according to the invention. Preferably the further semiconductor region ( 20 ) is recessed in the emitter region (1) and said emitter region ( 1 ) preferably comprises a lower doped part that borders on the base region ( 2 ) and that is situated below the further semiconductor region ( 20 ). The invention also comprises a method of manufacturing a semiconductor device ( 10 ) according to the invention.

The invention relates to a semiconductor device with a substrate and asemiconductor body of silicon comprising a bipolar heterojunctiontransistor with an emitter region of a first conductivity type, a baseregion of a second conductivity type, which is opposite to the firstconductivity type, and a collector region of the first conductivitytype, which are provided with, respectively, a first, a second and athird connection conductor, wherein the base region comprises asemiconductor material whose bandgap is smaller than that of thematerial of the collector region or of the emitter region.

The invention also relates to a method of manufacturing such a device.

Such a device and such a method are known from U.S. patent specificationU.S. Pat. No. 5,198,689, published on 30 Mar. 1993. Said documentdescribes a semiconductor device with a bipolar transistor having aheterojunction near the junction between the base region and thecollector region and near the junction between the base region and theemitter region, which is obtained by forming the base region from amixed crystal of silicon and germanium. Such a transistor has veryfavorable high-frequency properties, as evidenced, inter alia, by a highcut-off frequency f_(T).

A drawback of the known transistor, which manifests itself particularlyif the base region has a relatively high germanium content, is that ithas a comparatively low breakdown voltage from the emitter to thecollector, the so-termed BV_(ceo), which is undesirable.

Therefore it is an object of the invention to provide a semiconductordevice with a bipolar transistor which does not have said problem andwhich, despite the presence of germanium in the base region, has anacceptable emitter-collector breakdown voltage.

To achieve this, in accordance with the invention, a semiconductordevice of the type mentioned in the opening paragraph is characterizedin that the doping dose of the emitter region is locally reduced by afurther semiconductor region of the second conductivity type which isembedded in the emitter region.

In this case, “doping dose” is to be taken to mean the overall quantityof doping atoms per unit of area, i.e. the integral of the dopingconcentration across the thickness of the emitter region. At a constantdoping concentration, the doping dose is equal to the product (N×d) ofthe doping concentration (N) and the thickness (d) of the emitterregion. The invention is based on the recognition that the reduction ofthe BV_(ceo) in the known device is caused by the comparatively low basecurrent of the transistor, which entails a comparatively high currentgain of such a transistor. After all, the collector current of such atransistor is comparatively high, which is desirable for a very fasttransistor with a high f_(T). The invention is further based on therecognition that, in a device in accordance with the invention, the gainhas been reduced. After all, in the case of a state-of-the-art bipolartransistor with a monocrystalline emitter the base current, for an npntransistor, is determined mainly by the recombination of holes at thesilicon-conductor interface of the emitter. As this recombination speedat such a silicon-metal interface is very high, the base current isactually determined by the Gummel number which, in a first order, isproportional to the doping concentration and the thickness of theemitter or, if the doping concentration is not constant, proportional tothe doping dose of the emitter region, through which the holes mustcross. To increase the hole current it is thus required that either thethickness of the emitter region or the doping concentration, or both,are chosen to be comparatively small. The first option is not attractivebecause of the risk of so-termed spikes if the emitter is thin, thesecond option is not attractive either because a low-impedance contactrequires the doping concentration near the connection conductor to behigh. By providing a further semiconductor region of the secondconductivity type, i.e. the p-type in the case of an npn transistor, insuch a manner that it is embedded in the emitter region, a reduction ofthe doping dose is achieved without the above-mentioned drawbacksassociated with a small thickness of the emitter region or a low dopingconcentration of said region. Finally, an important advantage of adevice in accordance with the invention resides in that itshigh-frequency behavior still proves to be excellent.

In a preferred embodiment of a semiconductor device in accordance withthe invention, the further semiconductor region is recessed in theemitter region on a side of the emitter region that borders on the firstconnection conductor. Excellent results have thus been obtained and, inaddition, the device of this modification can be manufacturedcomparatively easily, for example, by means of an ion implantation toform the further semiconductor region.

Preferably, the emitter region comprises a first part having a highdoping concentration that borders on the first connection conductor, anda second part having a lower doping concentration that borders on thebase region and extends below the further semiconductor region. As aresult, on the one hand, a low-impedance contact of the emitter regionis ensured and, on the other hand, the base current is further reducedas the lightly doped part of the emitter region also contributes to thereduction of the Gummel number at the location of the furthersemiconductor region.

Good results are obtained if the further semiconductor region comprisesa number of sub-regions which are mutually separated by parts of theemitter region. The behavior of the transistor is thus determined in amanner that is as homogeneous as possible. The dimensions of thesub-regions of the further semiconductor regions preferably rangebetween 0.1 μm and 2 μm, and the parts of the emitter region situatedtherebetween preferably have dimensions in the range of 0.1 μm to 20 μm.

Preferably, the sub-regions of the further semiconductor region arejuxtaposed in the longitudinal direction of the emitter region. This toomakes the manufacture of a device in accordance with the inventioncomparatively simple. The emitter region is preferably situated at thesurface of the semiconductor body so that, for the formation of thefurther semiconductor region, use can be made of an accurate,reproducible and simple technique such as ion implantation. The bestresults are obtained if the first conductivity type is the n-type.Implantation of the further semiconductor region can be carried out, forexample, by means of boron ions.

In a favorable embodiment of a device in accordance with the invention,the germanium contact of the base region ranges between 10 and 30 at. %,and is preferably approximately 20 at. %.

A method of manufacturing a semiconductor device with a substrate and asemiconductor body of silicon comprising a bipolar transistor with anemitter region of a first conductivity type, a base region of a secondconductivity type, which is opposite to the first conductivity type, anda collector region of the first conductivity type, which are providedwith, respectively, a first, a second and a third connection conductor,wherein the base region is provided with a semiconductor material thebandgap of which is smaller than that of the material of the collectorregion or of the emitter region, is characterized in accordance with theinvention in that the doping dose of the emitter region is locallyreduced by a further semiconductor region of the second conductivitytype which is embedded in the emitter region.

Preferably, the further semiconductor region is formed so as to berecessed in the emitter region from the surface of the semiconductorbody. In a favorable modification, the emitter region is formed so as tocomprise a first part having a high doping concentration which borderson the first connection conductor, and a second part having a lowerdoping concentration which borders on the base region, the second partbeing formed below the further semiconductor region.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiment(s) described hereinafter.

In the drawings:

FIG. 1 is a diagrammatic, perspecpective view of an embodiment of asemiconductor device in accordance with the invention,

FIG. 2 is a diagrammatic, cross-sectional view at right angles to thethickness direction and taken on the line II-II of the device shown inFIG. 1,

FIG. 3 shows the rated base current (I_(B)) and the maximum cut-offfrequency f_(T) as a function of the thickness (d) of the part of theemitter region of the device shown in FIG. 1 that is situated below thefurther semiconductor region, and

FIG. 4 shows the rated base current (I_(B)) and the maximum cut-offfrequency f_(T) as a function of the doping concentration (n) of thepart of the emitter region of the device shown in FIG. 1 that issituated below the further semiconductor region, and

FIGS. 5 through 11 are diagrammatic, cross-sectional views at rightangles to the thickness direction, or in perspective as in FIG. 11, ofthe device of FIG. 1 in successive stages of the manufacture by means ofa method in accordance with the invention.

The Figures are not drawn to scale and some dimensions are exaggeratedfor clarity. Corresponding regions or parts are indicated by means ofthe same reference numerals whenever possible.

FIG. 1 is a diagrammatic, cross-sectional view at right angles to thethickness direction, of an embodiment of a semiconductor device inaccordance with the invention. FIG. 2 is a diagrammatic, cross-sectionalview at right angles to the thickness direction and taken along the lineII-II of the device shown in FIG. 1. The device 10 of this examplecomprises (see FIG. 1) a semiconductor body 11 with a p-type siliconsubstrate 12 and a semiconductor layer structure provided thereon andwith a bipolar transistor. The, in this example discrete, transistor hasan n-type emitter region 1, a p-type base region 2 and an n-typecollector region 3, which are provided with, respectively, a first, asecond and a third connection conductor 4, 5, 6. The connectionconductor 4 of the emitter region 1 is shown in FIG. 2, but omitted inFIG. 1 for clarity. The base region 2 comprises a mixed crystal ofsilicon and germanium having a germanium content of 20 at. %. Thecollector 3 and the emitter 1 contain silicon. The collector 3comprises, in this case, a lightly doped part 3A, the so-termed driftregion, and a heavily doped part 3B that borders on the substrate 12.The connection conductors 4, 5, 6 of, respectively, the emitter region1, the base region 2 and the collector region 3 contain, in this case,aluminum. The emitter region 1 comprises, in this case, a first part(1B) that has a high doping concentration and borders on the connectionconductor 4, and a second part (1A) that has a lower dopingconcentration and borders on the base 2. The connection conductor 6 isconnected to the collector region 3 by means of a heavily dopedcollector connection region 3C that is recessed in the semiconductorbody 11. The device 10 further comprises different isolating regions 7,8, 9 which, in this case, contain silicon dioxide.

In accordance with the invention, the doping dose of the emitter region1 is locally reduced by a further semiconductor region 20 of the second,in this case p-, conductivity type which is embedded in the emitterregion 1. The p-type region 20 is, in this case, recessed at thelocation of the regions 20 in the most heavily doped part 1B of theemitter region 1 and reaches as far as the less heavily doped part 1A ofthe emitter region 1. As a result, the emitter region 1 locallycomprises parts 1A having a reduced thickness which are situated belowthe recessed further region 20. As a result, the doping dose which, at aconstant doping concentration, is the product of the thickness and thedoping concentration is locally reduced in the emitter region 1. Thisresults in a higher base current during operation of the device, as aresult of which the gain of the transistor is reduced. This is animportant advantage because the presence of a mixed crystal of siliconand germanium in the base region 2, on the contrary, leads to arelatively low base current and hence to an increase of the gain of thetransistor, which would in turn lead to a relatively low BV_(ceo), whichis undesirable. The increase of the base current as a result of themeasure in accordance with the invention will, entirely or partly,compensate for said reduction, as a result of which the undesirablereduction of BV_(ceo) in a device 10 in accordance with the inventiondoes not take place. The p-type further region 20 is formed, in thiscase, by a local ion implantation.

The lateral dimensions of the device 10 of this example are 4 μm×10 μm.The part 1A of the emitter region 1 has a doping concentration of 2×10¹⁸at/cm³ and a thickness of approximately 100 nm, the part 1B has a dopingconcentration of approximately 10²⁰ at/cm³ and a thickness ofapproximately 0.2 μm. The base region has a doping concentration ofapproximately 10^(19 at/cm) ³ and a thickness of 30 nm. The parts 3A, 3Bof the collector region 3 have a doping concentration of, for example,5×10¹⁷ at/cm³ and 10²⁰ at/cm³, respectively, and a thickness of 50 nmand 500 nm, respectively. The width of the emitter region 1 is 500 nm,in this case, while the length is 5 μm. The dimensions of the p-typeregions 20 are 500 nm×200 nm and the spacing between them is 200 nm. Thethickness of the further regions 20 is, in this case, approximately 0.2μm, so that the underlying part 1A of the emitter region 1 has athickness of 100 nm at said location, being the difference between theoriginal thickness of the emitter region 1 (0.2 μm+0.1 μm) and thethickness of the further region 20 (0.2 μm). The effect of the localreduction of the doping dose of the emitter region 1, i.e. the influencethereof on the base current I_(B), will be illustrated hereinafter bymeans of FIGS. 3 and 4.

FIG. 3 shows the rated base current (I_(B)) and the maximum cut-offfrequency f_(T) as a function of the thickness (d) of the part of theemitter region of the device of FIG. 1 that is situated below thefurther semiconductor region. FIG. 4 shows the rated base current(I_(B)) and the maximum cut-off frequency F_(T) as a function of thedoping concentration (n) of the part of the emitter region of the deviceof FIG. 1 that is situated below the further semiconductor region.Curves 31 of FIG. 3 and curves 41 of FIG. 4 show that by a localreduction of, respectively, the thickness d of the emitter region 1 bymeans of the further region 20 and of the doping concentration n of theemitter region 1, the base current I_(B) can be easily reduced by,respectively, a factor of 3 and a factor of 1.5. By reducing the dopingdose by locally reducing the thickness d as well as the dopingconcentration n, a base current increase by a factor of 4.5 (=3×1.5) canthus be readily achieved, as a result of which the gain is reduced by anequal factor of 4.5. A local reduction of the doping concentration canbe achieved, for example, by building up the emitter region 1 out of twoparts, i.e. a first part having a comparatively low doping concentrationthat borders on the base region 2, and a second part having acomparatively high doping concentration that borders on the connectionconductor 4.

Curves 32 and 42 in FIG. 3 and FIG. 4, respectively, show that thishardly involves an unfavorable influence on the speed of the device 10in accordance with the invention. Said curves 32, 42 show the variationof the maximum cut-off frequency F_(T) as a function of theabove-mentioned thickness d and doping concentration n. This means thatsaid frequency, which ranges between 140 and 150 GHz, changes hardly ifthe doping dose is locally reduced in the emitter region 1 in a device10 in accordance with the invention. The device 10 of this example ismanufactured, for example, in the following manner by means of a methodin accordance with the invention.

FIGS. 5 through 11 are diagrammatic, cross-sectional views at rightangles to the thickness direction, or in perspective as in FIG. 11, ofthe device of FIG. 1 in successive stages of the manufacture by means ofa method in accordance with the invention. For the starting material(see FIG. 5) use is made of a p-type silicon substrate 11 in which, bymeans of local ion implantation (the mask used is not shown), a heavilydoped part 3B is formed of the collector region 3 of the transistor tobe formed of the device 10 with the semiconductor body 11.

Subsequently (see FIG. 6) by means of epitaxy an n-type layer 3 isprovided that forms a drift region 3A of the collector region 3 to beformed. A connection region 3C of the collector region 3 is locallyformed therein (the mask used is not shown) by means of ionimplantation.

Subsequently (see FIG. 7) isolating regions 7, 8 of silicon dioxide areformed in the semiconductor body 11 by means of etching and (local)oxidation of the semiconductor body 11 of silicon. The masks used insaid process are not shown in the drawing. First of all the surface iscovered with a silicon dioxide region 7 by means of local oxidation.Next, grooves are etched which are turned into an isolating region 8, aso-termed STI (=Shallow Trench Isolation) region 8, by filling thegrooves using TEOS (=Tetra Ethyl Ortho Silicate). The parts of the oxidelayer deposited in this process that are situated outside (and above)the grooves are removed again by means of CMP (=Chemical MechanicalPolishing).

Subsequently (see FIG. 8) by means of epitaxy a p-type silicon layer 2is applied to which 20 at. % germanium is added, which layer will formthe base region 2, and an n-type silicon layer 1A is provided thereonwhich will form a less heavily doped part of the emitter region 1. Theparts of these layers 2, 1A situated above the isolation regions 7, 8are polycrystalline, the parts situated therebetween aremonocrystalline.

Next (see FIG. 9) parts of the layers 2, 1A that are situated outsidethe active region 3A are removed by means of photolithography andetching. The mask used for this purpose, which is made of for examplesilicon dioxide, is not shown in the drawing and is subsequently removedagain. Next the entire surface of the semiconductor body 11 is coveredwith an isolating layer 9 of silicon dioxide which is provided by meansof CVD (=Chemical Vapor Deposition).

Subsequently (see FIG. 10) an opening is formed in the isolating layer 9by means of photolithography and etching. Next, by means of CVD, ann-type silicon layer 1B is applied to the surface of the semiconductorbody 11 and patterned by means of photolithography and etching. Theresultant monocrystalline, n-type region 1B forms a more heavily dopedpart 1B of the emitter region 1.

Next (see FIG. 11) the entire surface of the semiconductor body 11 iscovered with a mask layer, for example of a photoresist, which masklayer is not shown. Openings are formed therein at the location of theregions 20 to be formed in the silicon region 1B, after which the p-typeregions 20 are formed by means of a boron ion implantation. A series ofp-type regions 20 is thus formed in the more heavily doped part 1B ofthe emitter region 1, as shown in FIG. 2. At the location of theseregions 20, the emitter region 1 thus has a smaller thickness and, inaddition, a relatively low doping concentration. Said boron ionimplantation is also used, in this case, to form connection regions 22for the base region 2. For this purpose, a suitable opening is formed inthe isolating layer 9 and, in addition, in the above-mentioned mask thatis not shown in the drawing.

Next (see FIGS. 1 and 2) a contact opening is formed in the isolatinglayer 9 at the location of the collector connection region 3C, afterwhich an aluminum layer is deposited from which the connectionconductors 4, 5, 6 of, respectively, the emitter region 1, the baseregion 2 and the collector region 3 are formed by means ofphotolithography and etching. Individual devices 10 which are ready forfinal assembly are subsequently obtained after a separation process suchas sawing.

The invention is not limited to the examples given herein, and withinthe scope of the invention many variations and modifications arepossible to those skilled in the art. For example, besides in a discretesemiconductor device, the invention is also excellently suited for usein an integrated semiconductor device such as a BICMOS (=BipolarComplementary Metal Oxide Semiconductor) IC (=Integrated Circuit).

It is further noted that instead of STI isolation regions, isolationregions obtained by means of the LOCOS (=Local Oxidation Of Silicon)technique can be applied. The structure of a device in accordance withthe invention may be formed so as to comprise one or more mesa-shapedparts but also so as to be (substantially) entirely planar. Anothermodification is obtained by realizing the desired variation of thebandgap in the emitter, base and collector by increasing the bandgap ofthe base region with respect to the collector region (or the emitterregion) instead of by reducing the bandgap of said region(s). Thecollector region (and the emitter region) may then contain, for example,silicon carbide while the base region contains silicon.

As regards a method in accordance with the invention it also appliesthat many variations and modifications are possible. For example, theheavily doped part of the emitter region may also be formed by means ofoutdiffusion from a so-termed PSG (=Phosphor Silicate Glass) layer orfrom a polycrystalline layer that forms part of the connection conductorof the emitter region. The base region may alternatively be formed bymeans of a BSG (=Boron Silicate Glass) layer or by means of VPD (=VaporPhase Doping). For the connection of the base region it is also possibleto use a doped polycrystalline Si.

1. A semiconductor device with a substrate and a semiconductor body ofsilicon comprising a bipolar heterojunction transistor with an emitterregion of a first conductivity type, a base region of a secondconductivity type, which is opposite to the first conductivity type, anda collector region of the first conductivity type, which are providedwith, respectively, a first, a second and a third connection conductorwherein the base region comprises a semiconductor material whose bandgapis smaller than that of the material of the collector region or of theemitter region, characterized in that the doping dose of the emitterregion is locally reduced by a further semiconductor region of thesecond conductivity type which is embedded in the emitter region.
 2. Asemiconductor device as claimed in claim 1, characterized in that thefurther semiconductor region is recessed in the emitter region on a sideof the emitter region that borders on the first connection conductor. 3.A semiconductor device as claimed in claim 1, characterized in that theemitter region (1) comprises a first part (1B) having a high dopingconcentration that borders on the first connection conductor (4), and asecond part (IA) having a lower doping concentration that borders on thebase region (2) and extends below the further semiconductor region (20).4. A semiconductor device as claimed in claim 1, characterized in thatthe further semiconductor region comprises a number of sub-regions whichare mutually separated by parts of the emitter region.
 5. Asemiconductor device as claimed in claim 4, characterized in that thedimensions of the sub-regions of the further semiconductor region lie inthe range of 0.1 μm to 2 μm, and the parts of the emitter regionsituated therebetween have dimensions in the range of 0.1 μm to 10 μm.6. A semiconductor device as claimed in claim 4, characterized in thatthe sub-regions of the further semiconductor region are juxtaposed inthe longitudinal direction of the emitter region.
 7. A semiconductordevice as claimed in claim 1, characterized in that the emitter region(1) is situated at the surface of the semiconductor body.
 8. Asemiconductor device as claimed in claim 1, characterized in that thegermanium content of the base region ranges between 10 and 30 at. %, andis preferably approximately 20 at. %.
 9. A semiconductor device asclaimed in claim 1, characterized in that the first conductivity type isthe n-conductivity type.
 10. A method of manufacturing a semiconductordevice with a substrate and a semiconductor body of silicon comprising abipolar transistor with an emitter region of a first conductivity type,a base region of a second conductivity type, which is opposite to thefirst conductivity type, and a collector region of the firstconductivity type, which are provided with, respectively, a first, asecond and a third connection conductor, wherein the base region isprovided with a semiconductor material the bandgap of which is smallerthan that of the material of the collector region or of the emitterregion, characterized in that the doping dose of the emitter region islocally reduced by a further semiconductor region of the secondconductivity type which is embedded in the emitter region.
 11. A methodas claimed in claim 10, characterized in that the further semiconductorregion is formed so as to be recessed in the emitter region from thesurface of the semiconductor body.
 12. A method as claimed in claim 10,characterized in that the emitter region is formed so as to comprise afirst part having a high doping concentration which borders on the firstconnection conductor, and a second part having a lower dopingconcentration which borders on the base region, the second part beingformed below the further semiconductor region.
 13. A method as claimedin claim 11, characterized in that the emitter region is formed so as tocomprise a first part having a high doping concentration which borderson the first connection conductor, and a second part having a lowerdoping concentration which borders on the base region, the second partbeing formed below the further semiconductor region.